The transmission method of ordinary phones is to directly transmit high-frequency signals to the other party. For example, if several pairs of phones are transmitted on the same line at the same time, they will interfere with each other. The carrier telephone modulates each audio signal or ringing current at the sending end to different transmission frequency bands, and transmits them to the receiving end through the line and the booster equipment, and then selects the required signals by their corresponding band-pass filters, and then reversely modulates them. Then it is restored to various audio signals or ringing currents. A filter is used to separate each channel of audio, so that each voice channel does not interfere with each other. Carrier telephone equipment can not only be used for multiple telephones, but can also reuse open carrier telegrams twice to transmit broadcast TV programs, faxes, and data transmission. There are two types of open-wire carrier telephone and cable carrier telephone. The voice channel capacity is divided into single channel, 3 channels, 12 channels, 24 channels, 60 channels, 120 channels, 960 channels, 1 800 channels and 10 800 channels.

High-capacity and medium-capacity carrier phones adopt multi-level modulation and demodulation methods. Generally, the front group, base group, super group, main group, super main group, etc. are formed through various levels of modulation. 3 channels are called the front group (Chinese regulations are 12~24 kHz); the international regulations 12 channels are called the base group (60~108 kHz); 60 channels are called the super group (312~552 kHz); the 300 channel is called the main group ( 812~2044 kHz); 900 channels are called super master groups and so on. The high-order group is composed of several low-order groups.